The microwave spectrum is generally defined for a range of frequencies between 0.3 and 1000 Gigahertz (GHz). For a frequency of use between 1 and 100 GHz, the term hyperfrequency is generally used. The hyperfrequency range is divided into several bands according to the various technical applications associated with them. These bands include the Q band, the frequency range of which is situated approximately between 30 and 50 GHz, and the V band, the frequency range of which is situated approximately between 50 and 75 GHz.
Hyperfrequency monolithic integrated circuits, also known by the name of MMIC (Monolithic Microwave Integrated Circuits) chip, are components used in electrical circuits having an application in the microwave field. These components are for example used in communication and navigation systems.
Each MMIC chip can include several circuits such as amplifier circuits, mixers or oscillators for example. An MMIC chip includes contact pads on its upper surface, around the edge, in order to provide the interface between hyperfrequency signals and low-frequency signals.
MMIC chips are generally mounted on a support surface, also called substrate, including metallization to ensure interconnection with the MMIC chip. The interconnection between the contact pads on the surface of the MMIC chip and the interconnection metallization of the substrate is generally achieved by wiring or microstrip.
At hyperfrequency, the connection between MMIC chips is generally made by means of wires or strips of gold. It should be noted that the invention does not apply to MMIC chips only, but more generally to active and/or passive components (for example planar filters, various transitions) present in microelectronic hybrid technology. The waveguide is a hollow mechanical part serving to propagate electromagnetic waves (the hyperfrequency signal) with a minimum of distortion, unlike planar devices and worse still, with a wired device that in this case has a considerable discontinuity, severely degrading the propagation of the wave. The main defect of these waveguides is their compatibility with components (active or passive) which are generally made using planar technology.
In hyperfrequency applications, a high interconnection density is required to allow the transmission of the requisite information. Moreover, the chips must be interconnected by means of connections that preserve the quality of the transmission line, i.e. which ensure the maintenance of the impedances of the transmission line and avoid any discontinuity causing undesirable reflections, and which are of relatively short length to minimize signal distortion.
The rise in frequency, notably in the Q and V bands, requires a considerable effort to be expended on the interconnection technology in order to limit adjustments that are generally expensive and difficult to implement. Moreover, the integration of hyperfrequency functions requires the use of heterogeneous technologies, i.e. components of different heights are integrated, which leads to consequent step heights which are unfortunately often crippling to the rise in frequency. More precisely, conventional interconnections between planar components can generate considerable electrical paths (for example up to 1 millimeter) with respect to the wavelength only in cases where the frequencies are lower.
Several interconnection technologies are known. Mention may notably be made of wired technology, which allows the connection of two components by wiring, but which has the consequence of severely limiting the bandwidth. It is also possible to use so-called “interposer” technology, but the interconnection this offers is not very reliable. Mention may be made of “flip chip” technology, which consists in flipping the chip in such a way that the contact surfaces are face to face. Flip chip technology poses technical and industrial problems that are hard to solve. Notably, control of the electromagnetic environment is problematic and spatial control and heat management are difficult. Moreover, flip chip technology does not allow for compensation for large step heights between components (typically greater than 100 μm).